import java.util.*;
import java.io.*;
public class mianji {
	public static void main(String[]args){
		Scanner in=new Scanner(System.in);
		int i=0;
		while(i==0){
		q:{System.out.print("a=");
		double a = in.nextInt();
		while(a<=0){
			System.out.println("error");
			System.out.print("a=");
			 a = in.nextInt();
			}
	System.out.print("b=");
	double b= in.nextInt();
	while(b<=0){
		System.out.println("error");
		System.out.print("b=");
		b= in.nextInt();}
	System.out.print("c=");
	double c=in.nextInt();
	while(c<=0){
		System.out.println("error");
	System.out.print("c=");
	 c=in.nextInt();}
	if((a+b)<=c||(a+c)<+b||(b+c)<=a)
		{System.out.println("wrong number,please check and putin again\n");
		break q;
		}	double d = 0.5*(a+b+c);
	double s=Math.sqrt(d*(d-a)*(d-b)*(d-c));
	System.out.println("S="+s);
		}
		System.out.println("if you want to continue,type 0,if not,type 1:");
		i=in.nextInt();
		}
		System.out.println("thank you");
	}
	}